1. Field of the Invention
The present invention relates to control of a synchronous motor, and especially relates to a control method for realizing a high precision and high performance driving device for a motor without sensors detecting the speed and the position of the motor.
2. Prior Art
The prior art for controlling a synchronous motor without detecting a magnetic pole position includes vector control type sensorless method which is based on vector control with speed and position sensors of a synchronous motor, and is provided with a magnetic pole position estimator 14P and a speed estimator 33 instead of using the speed and position sensors as shown in FIG. 21, and a method called V/F control for controlling a synchronous motor as an open loop shown in FIG. 22.
In the vector control type sensorless method shown in FIG. 21, the other part than a part for detecting magnetic pole position and a part for detecting speed is constituted as in vector control method with sensors.
In FIG. 21, 1 is a generator for xcfx89r*, which is a rotating speed command, 2P is a controlling device for a motor, 3 is a PWM generator converting a voltage command into PWM pulses, 4 is an inverter, 5 is a synchronous motor, 6 is a current sensor for the synchronous motor, 7 is a conversion gain for converting a mechanical angular frequency into an electrical angular frequency, 9 is a dq coordinate converter for converting three-phase AC into a value on a rotation coordinate, 10 is an Id* generator generating a d axis current command Id*, 12 is a voltage command calculator, 13 is a coordinate converter for converting a value on the dq coordinate into a three-phase AC value, 14P is a magnetic pole position estimator estimating a magnetic pole position of the motor, 16 is an adder for adding (subtracting) signals, 21P is a speed controller for adjusting Iq* such that an estimated speed value matches the speed command, 24 is a current controller for correcting the voltage commands Vdc* and Vqc* such that detected current values Idc and Iqc match the respective command values Id*, Iq*, and 33 is a speed estimator for estimating the rotating speed of the motor.
In FIG. 21, the magnetic pole position estimator 14P corresponds to a magnetic pole position sensor, and the speed estimator 33 corresponds to a speed sensor. The speed controller 21P and the current controller 24 are provided as in a vector control device with speed and position sensors, and adjust such that the speed and the current match the respective command values. This type of vector control type sensorless method is described in xe2x80x9cHeisei 12 National Conference of Industry Application Society of Institute of Electrical Engineers of Japan, Proceedings [III], No. 97, pp. 963-966 Position Sensorless Control for Permanent Magnet Synchronous Motor with Calculation for Directly Estimating Axis Errorxe2x80x9d. Another sensorless driving technology for a synchronous motor is disclosed in xe2x80x9cInterior Permanent Magnet Motor: Kaitani, Matsubara, Watarai, Mitsubishi Denki Giho, Vol. 73, No. 9, 1999, pp. 68-71xe2x80x9d
The V/F control does not have an automatic adjusting part for the speed and the current, and determines a voltage impressed on a motor directly from a speed command as shown in FIG. 22. 2Q is a V/F control device, 15 is a zero generator for always setting Vdc* to zero, and 125 is a power generating coefficient gain corresponding to a power generating coefficient Ke of the motor in FIG. 22. The V/F control does not estimates a magnetic pole axis as in the vector control type sensorless method, and provides a very simple control structure. However, if a load changes abruptly while driving, it may generate a transient vibration. To restrain the transient vibration, Japanese application patent laid-open publication No.2000-236694 discloses a method adding a control loop for correcting the speed from the detected current value.
The vector control type sensorless method includes a speed controller and a current controller, and adjusting the control gains for them to appropriate values exploits a control capability of a motor. For that purpose, a magnetic pole estimator and a speed estimator should fully function in place of a position sensor and a speed sensor. However, since an actual calculation for the estimation is affected by a parameter fluctuation of a motor, or a delay in the calculation, accuracy comparable to that of a position or a speed sensor is not attained, and an estimation error always accompanies.
An estimation error for a magnetic pole axis is described in FIG. 23. A magnetic pole axis in a motor is defined as d axis. An orthogonal axis to the magnetic pole axis is defined as q axis. Estimated axes in a controller are defined as dc axis and qc axis. An axis error AO exists between them. If the pole estimation makes the axis error to zero, the relationship described in FIG. 24 is attained, resulting in an ideal vector control. Here, xe2x80x9cidealxe2x80x9d means motor current is orthogonal to motor magnetic flux, and entire current component contributes to the torque.
However, an axial error exists in reality, and the vector control type sensorless method does not attain a sufficient capability for the speed control or the current control, and adjusting the control gains for them becomes difficult. When an unstable phenomenon arises, it is difficult to determine if the direct cause is the estimation error or the effect from the gain setting in the controller, thereby making the inquiry for the cause difficult. Also, for the vector control type senseless method, since a motor is driven at a high speed, a high speed calculation is required, and a low price microcomputer providing a low performance does not meet the requirement.
On the other hand, since the V/F control does not include parts requiring adjusting as in the vector control type sensorless method, it controls the speed of a motor variably without adjusting. However, the d and q axes do not match the dc and qc axes, it is difficult to attain an advanced control. A vector chart for the relationship between the voltage and the current for the V/F control is shown in FIG. 25. In the V/F control, a voltage axis is qc axis, and as the load increases, the axis error becomes larger accordingly. Thus, disturbance such as a load torque fluctuation may cause a problem such as a vibration or an over current.
The purpose of the present invention is to reduce the number of parts requiring adjusting by simplifying the control structure, thereby stabilizing the control system, resulting in providing a driving device for a motor having a comparable capability as the conventional vector type sensorless method.
A driving device for a synchronous motor of the present invention calculates voltages impressed on a motor on coordinate axes (dc/qc axes) based on the magnetic pole axis as in vector control. The driving device for a synchronous motor of the present invention does not include an automatic adjusting part such as a speed controller or a current controller, and a command value such as a rotation speed command and a current command is used for calculating a voltage command. Iq*, which corresponds to a torque current command, changes depending on a load condition of a motor, and is provided by calculating based on a detected current value.